1. Technical Field
The present invention relates to improved methods and apparatus for the determination of the total concentration of organic carbon compounds in aqueous process streams and in bulk solutions. The invention is especially adapted for use in measuring carbon in deionized water or deionized water with dissolved carbon dioxide, which is often used in the manufacture and processing of electronic components, fine chemicals and pharmaceuticals. The present invention in one embodiment includes the measurement of the temperature and conductivity of an aqueous sample, the oxidation of the organic components of the sample stream, and the sensitive and selective detection of carbon dioxide utilizing a selective carbon dioxide gas permeable membrane and conductometric detection to determine the level of organic carbon in the sample. In a preferred embodiment for some applications, accurate conductometric detection of carbon can be carried out utilizing a single conductivity cell.
2. Background Art
The measurement of the total organic carbon (TOC) concentration and total carbon (organic plus inorganic) concentration in water has become a standard method for accessing the level of contamination of organic compounds in potable waters, industrial process waters, and municipal and industrial waste waters. In addition to widespread terrestrial applications, the measurement of TOC is one of the primary means of determining the purity of potable and process waters for manned space-based systems including the space shuttle, the proposed space station and for future manned explorations of the moon and planets.
The United States Environmental Protection Agency recently promulgated new rules aimed at reducing the levels of disinfectant by-products in drinking water. Formed from the reaction of chlorine and other disinfectants with naturally occurring organic matter, disinfectant by-products are potentially hazardous compounds including trihalomethanes (CHCl.sub.3, CHBrCl.sub.2, etc.), haloacetic acids, and other halogenated organic species. The new rules also require monitoring the levels of natural organic material in raw water, during the treatment process and in the finished water by measurement of total organic carbon concentration.
Very pure water is used for the manufacture of electronic components, and also in certain processes involving fine chemicals and pharmaceuticals. The water required for such uses is often deionized and carbon based impurity concentration in the parts per billion or even parts per trillion range must be monitored.
A variety of prior art approaches for measuring the total organic carbon content of water have been proposed. For example, See U.S. Pat. Nos. 3,958,941 of Regan; 3,224,837 of Moyat; 4,293,522 of Winkler; 4,277,438 of Ejzak; 4,626,413 of Blades, et al. and 4,666,860 of Blades, et al.; and 4,619,902 of Bernard.
Representative of the devices described in these references are the methods disclosed in U.S. Pat. No. 3,958,941 of Regan. In Regan an aqueous sample is introduced into a circulating water stream that flows through a reaction chamber where the sample is mixed with air and exposed to ultraviolet (U.V.) radiation to promote the oxidation of organic compounds found in the sample to form carbon dioxide. The carbon dioxide formed in the reaction chamber is then removed from solution by an air stripping system and introduced into a second chamber containing water that has been purified to remove ionic compounds. The conductivity of the water in the second chamber is measured, and any increase in conductivity is related to the total concentration of carbon dioxide following oxidation in the first reactor. In Ejzak, persulfate is added to an aqueous sample stream prior to oxidation of the stream using ultraviolet radiation in a series of reactors. Ejzak also describes the use of an inorganic carbon stripping process--before oxidation of the organic carbon--that includes the addition of phosphoric acid to the sample stream. After oxidation, the sample stream is passed into a gas-liquid separator where the added oxygen acts as a carrier gas to strip carbon dioxide and other gases from the aqueous solution. In the preferred embodiment, the gas stream is then passed through an acid mist eliminator, a coalescer and salt collector, and through a particle filter prior to passage into an infrared (IR) detector for the measurement of the concentration of carbon dioxide in the gas stream.
The methods and apparatus disclosed by Ejzak provide improvements over the teachings of Regan; however, the Ejzak device requires extensive manual operation and is also generally unsatisfactory. The Ejzak device requires three external chemical reagents; oxygen gas, aqueous phosphoric acid and an aqueous solution of sodium persulfate. Both the phosphoric acid and persulfate solutions must be prepared at frequent intervals by the operator due to the relatively high rate of consumption. The Ejzak device requires dilution of the sample if the solution contains high concentrations of salts in order to ensure complete oxidation of the sample and to eliminate fouling of the particle filter located prior to the IR carbon dioxide detector. As with Regan, relatively large sample sizes are required--typically 20 .mu.L of sample for accurate measurement at 0.5 mg/L total organic carbon--and the carbon dioxide formed in the oxidation chamber is removed using a gravity dependent technique that cannot be easily used in space-based operations.
Another improved method and apparatus for the measurement of total organic carbon in water is disclosed in U.S. Pat. No. 4,293,522 of Winkler. In Winkler, an oxidizing agent, molecular oxygen, is generated in-situ by the electrolysis of water. Organic compounds are subsequently oxidized to form carbon dioxide by the use of U.V. radiation and the in-situ generated oxygen. The irradiation and electrolysis processes are both accomplished in a single oxidation chamber. Winkler does not teach that the aqueous sample stream be acidified to assist in the removal of carbon dioxide from solution, and in fact teaches against the use of acid. Therefore, this method and apparatus cannot be used for the measurement of organic compounds in basic aqueous samples. The oxidation chamber of Winkler uses a solid electrolyte to separate the two electrodes employed for the electrolysis of water. The solid electrolyte described by Winkler is composed of an organic polymer which, upon exposure to oxygen, ozone and U.V. radiation, will undergo limited oxidation to form carbon dioxide, therefore resulting in unacceptable background levels of organic compounds in the sample stream, particularly at low organic compound concentrations.
The Winkler patent describes a conductometric carbon dioxide detection system wherein the sample stream exiting the oxidization chamber is maintained in an equilibrating relationship with a stream of deionized water. The two flowing streams are separated by a gas permeable membrane that permits the concentration of carbon dioxide to equilibrate between the streams. The concentration of the carbon dioxide generated in the oxidation chamber is thereafter determined by measuring the conductance of the deionized water stream. However, the use of two continuously flowing and recirculating streams with separate pumps on either side of the membrane as taught by Winkler introduces precise operating parameters into the detection process that require frequent calibration adjustments, such as adjustments necessitated by ionic contamination from the circulatory pump. Using one pump for the sample stream and a different pump for the deionized water stream can produce varying differential flow rates which introduce additional errors into the system. The use of a membrane as taught in the Winkler patent allows the passage of acid gases other than carbon dioxide, thereby interfering with the measurement of carbon dioxide. The device described in Winkler uses a large volume batch process which would also be very time-consuming to operate, to the point where it would not be practical for commercial use.
Another TOC detector of the prior art is disclosed in U.S. Pat. No. 4,619,902 of Bernard, which teaches the oxidation of organic compounds to form carbon dioxide using per sulfate oxidation at elevated temperatures--typically 20 to 100.degree. C.--in the presence of a platinum metal catalyst. Bernard recognizes that the materials used in the construction of instrumentation for the determination of total organic carbon in water can contribute organic compounds to the sample during the measurement process, and teaches that inert materials such as PTFE must be used to reduce this background from the measurement. As with certain of the previously mentioned disclosures, a gas stripping technique is employed to collect the formed carbon dioxide, and measurement is made using IR spectrometry. Bernard also recognizes that aqueous solutions of sodium persulfate are relatively instable due to auto-degradation of the reagent.
An additional system for the measurement of organic compounds in deionized water is disclosed in U.S. Pat. No. 4,626,413 of Blades and Godec. The apparatus described by Blades and Godec utilizes direct U.V. oxidation of organic compounds to form carbon dioxide, which is measured by using conductometric detection during the oxidation process. In the apparatus and method described in the Blades and Godec patent, the oxidation of some organic compounds containing halogens and other heteroatoms will lead to the formation of strong acids such as HCl, H.sub.2 SO.sub.4 and HNO.sub.3, which interfere with the conductometric method employed. The TOC detector described in the Blades and Godec patent operates in a batch mode where the oxidation can be unpredictable and can be very long (greater than 25 minutes). This does not allow this device to be an effective real time analyzer. The sample chamber in the device is relatively large, and large amounts of sample must be flushed through the chamber between evaluations, and is therefore difficult to calibrate using normal chemical standards. The Blades device is also limited to the measurement of total organic compounds in deionized water and cannot be used for samples containing ionic compounds other than bicarbonate ion.
In U.S. Pat. No. 4,209,299 of Carlson, it is disclosed that the concentration of volatile materials in a liquid can be quantitatively determined by transferring the desired material through a gas permeable membrane into a liquid of known conductivity, such as deionized water. The Carlson patent describes the measurement of a number of volatile organic and inorganic compounds, but does not suggest the combination of the method or process in conjunction with a carbon dioxide producing reactor. The Carlson patent does not teach the use of a selective membrane to limit the measurement to the gas of interest.
An improved carbon analyzer is disclosed in related U.S. Pat. No. 5,132,094 by Godec et al. (See also related U.S. Pat. No. 5,443,991.) The Godec patents are commonly assigned with the present invention, and are herein incorporated in their entirety by this reference. Originally developed for NASA, the device described in the Godec patents uses UV/persulfate oxidation and a novel CO.sub.2 detection technique utilizing a selective gas-permeable membrane and a conductivity cell. The gas-permeable membrane is used to separate the acidified sample stream (pH&lt;2) from a thin layer of deionized water. A solenoid valve is opened to allow fresh deionized water to flow into the membrane module and the solenoid valve is closed. Carbon dioxide formed from the oxidation of organic compounds will selectively diffuse across the membrane into the deionized water, where a portion of the CO.sub.2 will ionize to produce H+ and HCO.sub.3.sup.- ions. After an equilibration period, the solenoid valve is opened to flush the solution into a conductivity and temperature measurement cell, and the concentration of CO.sub.2 in the deionized water is determined from the conductivity and temperature measurements.
The selective membrane-based conductivity detection of carbon dioxide in a TOC detector as taught in the Godec patents offers several advantages to other methods. Calibration is extremely stable, and the calibration c(an be easily performed by the analyst. No purge gases are required. The technique is highly selective for carbon dioxide because of the use of a selective carbon dioxide gas permeable membrane which is extremely sensitive, permitting detection of TOC down to parts per trillion levels. It also has a wide dynamic range, permitting measurement of up to at least 50 ppm TOC without sample dilution.
In operation of one of the preferred embodiments taught in the Godec patents, the sample is drawn into an analyzer by means of a peristaltic pump, and two reagents are added via syringe pumps. Acid (6 M H.sub.3 PO.sub.4) is added to reduce the pH of the sample stream and ammonium per--sulfate (15% (NH.sub.4).sub.2 S.sub.2 O.sub.8) is added to assist in the oxidation of organic compounds in the sample stream. The sample stream is split for measurement of inorganic carbon (IC) concentration (IC.dbd.[HCO.sub.3 -]+[CO.sub.3 -.sup.2 ]+[CO.sub.2 ]) without oxidation, and the measurement of total carbon (TC concentration after oxidation of the organic components in the sample to carbon dioxide. TOC is then computed from the difference (TOC.dbd.TC--IC). For samples containing high levels of inorganic carbon and lower levels of TOC, the Godec patents teach an embodiment of the device where an IC removal module is used to remove the inorganic carbon and permit accurate TOC measurements.
Supplies of the acid and oxidizer may be pre-packaged and stored in the analyzer, eliminating the need for reagent preparation by the analyst. Deionized water is continuously produced in the analyzer using a mixed-bed ion exchange resin with a capacity for several years of operation. The maintenance required is replacement of the reagent containers several times a year, replacement of the UV lamp and replacement of the pump tubing. The ease of use, low maintenance requirements and dependable performance have made this device the TOC analyzer of choice for monitoring water purification systems in semiconductor manufacturing, the pharmaceutical industry and both conventional and nuclear power plants.
Some industries routinely use, deionized water for various manufacturing and processing steps. In such industries, the accurate measurement of extremely low levels of TOC is often highly desirable. When the sample stream is known to be deionized water, the TOC detector described in the Godec patent contains several elements that are unnecessary and that make it less desirable for use in continuous rather than batch analysis of TOC.
For example, the Godec device requires the use of chemical reagents to acidify the sample stream and to assist in the oxidation process. One of several important aspects of the present invention is the recognition that TOC analysis of deionized water samples does not require all the processing and pretreatment steps of TOC devices found in the prior art.
A number of older as well as some very recent patents also address various approaches to the problem of determining the heteroorganic content of water or other substances. Of particular interest in this area is detecting and measuring the various halogenated hydrocarbons. Thus, it is becoming increasingly important in such diverse applications as semiconductor chip manufacturing and drinking water to be able to detect extremely low levels of halogenated hydrocarbons, such as the trihalomethaines (THMs). The EPA now regulates the permissible level of halogenated hydrocarbons in drinking water.
U.S. Pat. Nos. 5,480,806 (Duve) and 5,531,965 (Duve) teach processes in which organic and organic heteroatoms in gases are oxidized in a U.V. reactor followed by measurement of conductivity changes. The processes and apparatus of these patents are relatively complex, do not address measurements in liquids, and cannot differentiate organic and heteroatom content. U.S. Pat. No. 5,521,510 (Schunck et al.) teaches running water through an ion exchanger, splitting the streams, and measuring background conductivity in one of the streams. The second stream is heated to high temperatures so that organic carbon and organic heteroatoms can be determined by a thermal oxidation technique. U.S. Pat. No. 5,073,502 (Steele) describes a device that removes inorganic halogen salts with a sorbent bed, then oxidizes the sample using a U.V. reactor, and finally measures the solutions for halogens using ion chromatography. U.S. Pat. No. 5,081,047 (Steele et al.) uses an ion chromatograph to determine CO.sub.2, organic acids and carbonate levels in a water sample in conjunction with a UV reactor. U.S. Pat. No. 5,028,543 (Finch et al.) describes a method for extracting PCBs from soil samples followed by water removal and analysis with a color changing titration. U.S. Pat. No. 5,429,946 (Baccanti) teaches feeding samples containing halogen, phosphorous or sulfur into a continuous heated flash combustion reactor where they are oxidized in the presence of oxygen, followed by continuously monitoring the products of the combustion reactor. U.S. Pat. No. 5,427,955 (Shattuck et al.) describes al method of detecting halogenated organics in soil or oil using color producing reactions between a photodonor reagent and a halogenated organic compound. Changes in the optical absorption of the light-exposed photodonor are said to be proportional to the halogenated organic compound content. U.S. Pat. No. 4,160,802 (White et al.) teaches a process wherein a solid or liquid sample is combusted at high temperatures in an oxygen-hydrogen flame. Reaction products are then absorbed onto a film containing a colorimetric reagent solution. U.S. Pat. No. 5,106,754 (Steele et al.) uses expandable bellows to control expansion of gases during thermal combustion oxidation of the organics. The products of the combustion are sent to an infrared analyzer. U.S. Pat. No. 4,822,744 (Bellows) describes the use of a cation conductivity and chloride monitor before and after oxidation in a steam generator in a power plant. Carbon dioxide, organic anions and heteroatom anions (including chloride) are measured with the cation conductivity detector. This method may be used in conjunction with an ion chrolmatograph to measure other possible heteroatoms. U.S. Pat. No. 4,251,220 (Larson et al.) teaches measuring the conductivity of a water sample at elevated pressure and temperature conditions as part of a procedure for detecting NH.sub.4.sup.+, HCO.sub.3.sup.-, CO.sub.3.sup.-2, organic acids and heteroatom salts. The difference between two signals produces a measurement that is an approximation of the amine or ammonia concentrations in the sample. Finally, U.S. Pat. No. 4,801,551 (Byers et al.) describes a device to measure CO.sub.2 in high purity water (e.g., power plant water loops) by measuring the conductivity after passage through a cation resin bed at two different temperatures.
All of the foregoing devices and techniques have various limitations or drawbacks. The apparatus and methods of the present invention overcome some or all of these problems by providing a relatively easy to construct and easy to use system able to detect and measure various carbon contaminants in water with an extremely high degree of accuracy.